If a greenhouse is on your list for next spring, the part to do now is the part underneath it. A greenhouse is only as square, level and steady in the wind as the base it sits on, and base work is ground work: digging, levelling, compacting gravel, framing or pouring. That gets hard once the ground freezes. Build the base this fall, and in spring your kit goes straight onto a foundation that is ready for it.
Why fall is the right time
Concrete should never be poured on frozen ground, and a gravel pad cannot be properly compacted once it freezes. Fall gives you workable ground and time to do the job well, instead of rushing it in spring when you would rather be planting. Freeze and thaw can shift a pad over the winter, so check that it is still level before you start assembly.
What you can build on
Palram-Canopia lists four foundation types for its greenhouses: concrete, pressure-treated wood, a wood deck and paver blocks. Whichever you choose, it has to be solid, level and flat under the entire perimeter of the greenhouse base.
- Concrete. A slab or a perimeter footing gives the firmest, most permanent base.
- Pressure-treated wood frame. A good choice on uneven ground, as long as the lumber is wide enough to carry the base. More on that below.
- Wood deck. Works if the deck is level and solid under the whole base.
- Patio stones or pavers. Only on a well compacted gravel pad. See the patio stone section below.
How big it needs to be: the 2-inch rule
Palram-Canopia's installation manuals call for at least 2 inches (5 cm) of solid foundation outside the base rail and 2 inches inside it, on every side. On our aluminum-frame greenhouses, including the Harmony, Hybrid, Mythos, Balance, Americana and Glory, the galvanized steel base rail is about 1 inch wide. So wherever the rail sits, you need a bearing strip at least 5 inches wide.
- Find the base dimension in your greenhouse's assembly manual and add 4 inches to both the length and the width. That is your minimum foundation size.
- On a full concrete slab, the inside edge takes care of itself.
- On a wood frame, the lumber has to be wide enough. A 6x6 measures 5 1/2 inches and carries the rail on its own. A 4x4 measures only 3 1/2 inches and is too narrow, even when the outside dimensions are right.
Two inches is a minimum on each side, not a target. More is fine, as long as nothing overhangs an edge.
Our Rion resin greenhouses (EcoGrow, Grand Gardener and Prestige) need at least 2 inches of foundation beyond the base on every side. Rion's optional resin mounting base kits add up to 12 cm of height and extra stability, but they do not replace a level, solid foundation underneath.
Patio stones: the gravel does the work
Patio stones can make a good base, but the stones are not what carries the greenhouse. The well compacted gravel pad underneath them is. If that pad is soft or uneven, the stones settle and the greenhouse goes out of level and out of square.
- Build the gravel pad first, flat and level across the whole footprint. As general landscaping practice, not a Palram-Canopia specification, that usually means 4 to 6 inches of compacted 3/4 inch crushed gravel, placed and compacted in layers.
- Lay the stones tight and continuous under the entire base rail. Do not space them out like deck piers, or the rail spans unsupported gaps.
- Keep at least 2 inches of solid stone outside the rail and 2 inches inside it.
Stones laid on levelled soil, on sand alone or on an uncompacted base are not a foundation.
Anchoring
A greenhouse also has to be fixed down against the wind, and what comes in the box depends on the model. The Glory and the Victory Orangery include an anchoring kit. For the Harmony, Mythos, Hybrid, Balance and Essence, we carry Palram-Canopia's anchoring kit separately. Check your manual for how your model fastens to the foundation, and have the right fasteners for concrete or wood on hand before you build.
Your fall to-do list
- Choose the spot. Our guide on where to locate your greenhouse can help.
- Open your model's product page, download the assembly manual from the Download tab and note the base dimension.
- Mark out the foundation at the base size plus 4 inches each way.
- If you plan to heat the greenhouse in winter, work out how power will get there before the pad goes in, and have any wiring done to code by a licensed electrician.
- Build it level and square, then check it again in spring before assembly.
One of our customers built this raised gravel pad inside a rock wall, and laid a line for future power and water before the greenhouse went up.
Need a hand?
Our foundation guide walks through each foundation type, with diagrams. If you cannot find your base dimension, call us at 1-877-322-5487 and we will read it out of the manual for you. Not sure which greenhouse to plan for? Our greenhouse quiz points you to the models that fit. We are a Canadian family business and a certified Palram-Canopia dealer since 2007.